1. Field of the Invention
The present invention relates to a method of producing a ceramic laminate by laminating a plurality of ceramic layers.
2. Description of the Related Art
A ceramic laminate is often used as a high-performance component of, for example, a piezo-electric actuator. The piezo-electric actuator employs a ceramic laminate that is constituted by alternately laminating ceramic layers and internal electrode layers. In recent years, for instance, the piezo-electric actuator has been built up with an increased number of ceramic layers each of which having a small thickness to obtain a large displacement with a low voltage. For example, the piezo-electric actuator has a structure of a laminate of, generally, 50 to 700 layers by alternately laminating thin piezo-electric ceramic layers each having a thickness of usually 20 to 200 μm and metallic electrodes (internal electrodes), yet suppressing the overall size thereof for easy incorporation in an apparatus.
As a method of producing the above-mentioned laminate, International Patent Publication No. 2000-500925 teaches an art comprising cutting or punching first laminates obtained by laminating and temporarily adhering the sheets, dewaxing the first laminates and, then, stacking the laminates followed by sintering.
However, a decrease in the thickness of the ceramic layers and an increase in the number of the layers bring about such problems as delamination (peeling among the layers) and the occurrence of cracks in the ceramic laminate. The delamination and the cracks can cause a malfunction of the ceramic laminate.
The above-mentioned International Patent Publication No. 2000-500925 proposes forming first laminates of green sheets having a large area and a thickness of up to 3 mm, dewaxing the first laminates, and stacking them to form a second laminate of a total height of not smaller than 5 mm. This method decreases irregularity in the stress at the time of adhesion with pressure. However, the first laminates after dewaxed are very brittle and are likely to be damaged even by a small stress when they are taken out from the dewaxing furnace, conveyed and stacked. The delamination and cracks occur at the damaged portions in the subsequent final products.
As it is not limited to the case of piezo-electric actuators, this kind of problem could similarly occur in any other ceramic laminate.
A working example of the above publication further discloses one that is cut into a square shape. When punched into a square shape, however, the internal electrodes are extremely deformed at the corners, and the distance becomes nonuniform among the internal electrodes.
The cause of this deformation is attributed to the punching stress in the first laminate not being uniform because the first laminate being punched is thick and the viscosity is not the same between the internal electrodes and the piezo-electric sheets.
This deformation develops delamination (peeling of the layers) during the dewaxing and sintering, resulting in a change in the characteristics of the product because a uniform voltage is not applied. Therefore, the deformed portions must be cut off in some step after the punching, such as after the sintering. Many steps are required for cutting off the deformed portions, and the material that is cut off is wasted.
The deformation has a correlation to the warping of the work, and it is considered that the deformation could be decreased if the warping could be decreased.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 162364/1996 teaches an art of cutting a sheet, laminating a plurality of pieces of the sheets thus cut to form a first laminate, laminating and adhering another plurality of sheets on the first laminate with the application of pressure, and repeating the above-mentioned procedure a plural number of times until a finally desired number of pieces of sheets are laminated.
According to the method of the above publication, however, the sheet is cut into a small size. Therefor each piece of sheet must be handled while being conveyed and laminated, requiring a greatly extended period of time and deteriorating the production efficiency.